(a) Field of the Invention
The present invention relates to a liquid developer for use in electrostatic photography, which developer contains wax and a filler for preventing transfer-crush.
(b) Description of the Prior Art
The wet developing method which comprises developing electrostatic latent images formed on electrophotographic sensitive materials, electrostatic recording materials and the like with a liquid developer (which normally comprises dispersing toner particles consisting essentially of a colorant and a resin in a highly insulating carrier liquid) is profitable in that the etching effect is small and so copied images of high resolving power can be obtained. The transfer method which comprises forming images on the above mentioned materials and thereafter transferring said images onto another substrate is profitable in that common paper and plastic film can be used as the transfer sheet, and so, the copies are easy to handle. Accordingly, it may be said preferable to form copies making use of the combination of the wet developing method with the transfer method. However, this system involves the questions that in case a transfer sheet of low smoothness is used, the solid area in the transfer image deteriorates in uniformity as copying is repeated, while in case a transfer sheet of high smoothness is used, the transfer image deteriorates in sharpness as copying is repeated.
Therefore, various proposals have usually been made concerning the liquid developer in order to solve these questions. For instance, it is known that wax should be contained in the toner particles for the purpose of improving the uniformity of the solid image and a spherical substance such as glass beads and a spacer such as polymethacrylate particles should be mixed with and included among the toner particles as the filler for preventing transfer-crush for the purpose of improving the sharpness of the transfer sheet (for instance, Japanese Laid Open Patent Application No. 34328/1974 corresponding to U.S. Pat. No. 3915874 discloses the use of the 0.5-15.mu. spherical substance and Japanese Laid Open Patent Application Nos. 178252/1982, 200049/1982 and 298351/1983 disclose that the spacer whose particle diameter is 20-70.mu. is used as a rule). However, these developers have merits and demerits respectively. The developer comprising the wax-containing toner particles can not achieve the effect of improving sharpness, while the developer which includes the filler for preventing transfer-crush therein can not obtain the uniformity of the solid image. In the latter developer, furthermore, in case the spherical substance is used, if the amount of the spherical substance used is in excess, and in case the spacer is used, if its particle diameter is too large, not only the effect of improving sharpness can not be obtained but also the deterioration in image density is brought about, and the image uniformity is sometimes reduced according to circumstances.